The present invention relates to a matrix screen, its production process and a matrix display means with several tones, controlled on an all or nothing basis and incorporating such a screen. It is used in optoelectronics and particularly in the analogue display of complex images or in the display of alphanumeric characters, said displays being either monochrome or polychrome.
Information processing and telematic consoles, such as for example electronic telephone directories and microcomputers are becoming objects of everyday life. Most of these equipments which are presently available are equipped with cathode ray display tubes. However, other display means, such as e.g. flat matrix screens are increasingly replacing the cathode ray tubes, which are heavy, cumbersome and visually uncomfortable. Some of these flat screens display the formation of imagess and diagrams in several tints and even in color.
The invention more particularly relates to a flat matrix screen constituted by a material having optical properties which can be electrically modified, which is placed between a first group of p row electrodes formed from parallel conductive strips and a second group of q column electrodes formed from parallel conductive strips. The row and column electrodes cross one another, so that an image point x.sub.ij of the screen is defined by the overlap region of one row electrode i and one column electrode j, in which i and j are integers such that 1.ltoreq.i.ltoreq.p and 1.ltoreq.j.ltoreq.q. Means for supplying electrical signals on each electrode are provided in order to electrically modify the optical property of the material, in accordance with two different states. Numerous flat matrix screens of this type are known, which use as the sensitive material an electroluminescent material. This material is compatible with the display in half-tones or several tones, as well as colour displays. Such matrix screens are more particularly described in an article in IEEE Transactions on Electron Devices, vol ED-30, No 5, May 1983, pp 460-463 entitled "Thin Film Electroluminescent Devices: Influence of Mn-Doping Method and Degradation Phenomena".
Although the invention more particularly applies to such matrix screens, it also applies in more general terms to all display screens having a material, whereof one optical property can be modified with the aid of an electrical excitation. This material can be solid or liquid, amorphous or crystalline. The optical property can be an opacity, refractive index, transparency, absorption, diffusion, diffraction, convergence, rotary power, birefringence, intensity reflected in a given solid angle, etc.
The generally used electroluminescent matrix screens operate on all or nothing basis, i.e. they only permit a display in two tones, e.g. black and white. Such a matrix screen is more particularly described in FR-A-2 489 023. Their advantage is the use of relatively simple control or addressing integrated circuits.
In order to permit a display in several tones or half-tones, e.g. different grey tones, various electronic processes have been envisaged. These processes based on the application of different electrical signals as a function of the half-tone which it is wished to obtain, require the production of relatively complex integrated control circuits, whose cost, related to a column electrode of the matrix screen, is six times higher than the cost of a control operating on an all or nothing basis. In view of the number of row electrodes and column electrodes, the total cost of control circuits is prohibitive.